Highly elastic adhesive films for temporary bonds which can be re-released by pulling in the direction of the bond plane are known and are obtainable commercially under the designation "tesa Power-Strips". Bonds produced therewith offer a powerful hold and yet can be released again without trace and without damage to the substrate or to the adherends, as is described in DE 33 31 016 C2.
DE 37 14 453, DE 42 22 849, DE 42 33 872, DE 43 39 604, DE 44 28 587, DE 44 31 914 and DE 195 11 288 describe specific embodiments and applications of prior adhesive films.
The adhesive films described in these documents are subject to stringent requirements, for instance:
For flawless functioning they must offer a degree of finger tack (for light bonding=low applied pressure and for immediate loading) and bond strength (throughout the application) which are sufficient for the application in question. PA1 Prerequisites for the detachment process are high extensibility coupled with low offset yield stress and a tear strength which is high in comparison with the detachment force (stripping force). PA1 A marked reduction in finger tack on stretching is advantageous for the detachment process. PA1 For longer-term bonds, an appropriate ageing resistance in the bond joint is essential. PA1 Under high mechanical stresses (high shear loads and tip-shear loads) there may, if unsaturated styrene block copolymers (styrene-isoprene, styrene-butadiene block copolymers) are used, be development of ozone cracks in the adhesive and thus inward tearing of the adhesive films during the detachment process or detachment of the bonded article during the application period. PA1 Especially in the case of bonds which are subject to high shear loads and tip-shear loads, at application temperatures&gt;about 35.degree. C. a very high thermal shear strength is essential for sufficient bond strength. PA1 Many applications desire that the adhesive film be pigmented. The use of pigments, for example TiO.sub.2, may, however, have adverse effects on the bond strength, especially when used at high concentrations. PA1 Unlike single-layer systems, multilayer systems permit the production of adhesive films with differential (asymmetric) adhesion properties. By this means it is possible to realize systems whose adhesion properties are tailored to the substrates used, in terms of bonding performance and redetachability without residue. By using fillers in one layer of a two-layer adhesive film it is thus possible, for example, to obtain a controlled level of reduction in the adhesion (adhesive force) of one side of the adhesive film. By using block copolymers with a high block polystyrene content and/or a high glass transition temperature of the elastomer blocks in an outer layer it is possible to realize systems which on one side show normal contact adhesion (normal contact adhesion=contact adhesion at room temperature) while the second adhesive film surface features sufficient tack for a good adhesive bond only after heat activation. PA1 By exclusive addition of colour pigments to the middle layer of three-layer adhesive films, or to one of the middle layers in the case of adhesive film laminates with more than three layers, it is possible substantially to eliminate the influence of the pigments on the adhesive properties of the -adhesive films. In many cases it is possible by this means to obtain multilayer adhesive films which in comparison with pigmented, single-layer adhesive films permit a significant increase in the bond strengths with a virtually identical visual appearance of the adhesive films. PA1 By using styrene-ethylene/butylene or styrene-ethylene/propylene or styrene-butadiene block copolymers in the middle layer and styrene-isoprene-styrene block copolymers in the outer layers it is possible to obtain three-layer adhesive films characterized by high adhesion and by low propensity to tear on redetachment even after ozone exposure. PA1 Additions of polyvinyl acetates or ethylene-vinyl acetate copolymers or of reinforcing fillers, for example silica (e.g. Aerosils), to pressure-sensitive adhesive compositions based on styrene block copolymers generally bring about a marked reduction in the finger tack while significantly increasing the ozone resistance. Through the exclusive addition of corresponding compounds to the middle layer it is possible likewise to obtain adhesive films characterized by high adhesion and by low propensity to tear on redetachment even after ozone exposure. PA1 By varying the polymer concentration, the diblock content, the nature of the elastomer block, the block polystyrene content and the molar mass of the block copolymers used, and also the thickness of individual layers, it is possible to a large extent to control the mechanical properties of the novel adhesive films, especially the offset yield stress as a function of the degree of stretching. By this means it is possible to control the redetachment properties in a manner very largely independent of the top layers. Thus it is possible, by controlled variation of the block polystyrene content and/or of the diblock content and/or of the amount of styrene block copolymer in the middle layer and/or the relative thickness of the top layers to the middle layer, given a constant overall thickness of the adhesive films, to control to a large extent the force required for detachment. By using styrene block copolymers with a high diblock content and/or a low block polystyrene content in the middle layer and using styrene block copolymers having a high block polystyrene content and/or a low diblock content in the top layers it is thus possible, for example, to realize three-layer adhesive films which couple high bond strength with a low stripping force. PA1 By using highly ozone-resistant styrene block copolymers in the outer layers it is possible to realize ozone-resistant multilayer films using even block copolymers of low ozone resistance as middle layer. When using, say, SIS block copolymers with a low block copolymer content and/or a high diblock content for the middle layer it is thus possible, for example, to realize adhesive films of substantially improved ozone resistance, which because of the low Shore hardness of the middle layer at the same time show a high degree of conformity to rough substrates. PA1 By using styrene block copolymers with a high triblock content and/or a high block polystyrene content in the first layer and with a high diblock content and/or low block polystyrene content in the second layer it is possible to realize adhesive films having improved thermal shear strength and high contact adhesion on one side. Such systems are particularly suitable when bonding to substrates which have surface temperatures&gt;&gt;room temperature.
The abovementioned adhesive films consist of only a single layer of adhesive which is required to meet, sufficiently, all of the abovementioned properties for a large number of applications. In this case, varying the formulation of the adhesive layer influences totally a number of these properties. Many product properties, moreover, are in mutual contradiction, and some are almost totally mutually exclusive. The formulation of the adhesives is accordingly complex, with many combinations of properties being impossible to establish to a sufficient degree if at all.
Against the background of this state of the art, the aim of the present invention was to provide highly elastic adhesive films which permit extensively independent control and broad formulatability of individual product properties. This is achieved through the use of adhesive film laminates which consist of a plurality of adhesive layers. It is possible optionally to integrate, between the individual adhesive layers, diffusion barrier layers, which act as diffusion barriers for migratable formulation constituents in adjacent adhesive layers. Through the multilayer structure it is possible to control in a targeted manner and separately from one another the overall properties of novel adhesive film laminates, by varying, for example, the composition (formulation), the thickness and the number of the individual layers, and thus to produce a spectrum of properties denied to the known single-layer systems. In this arrangement, the profile of properties of individual adhesive film layers can be controlled by the nature and amount of the formulation constituents used (polymers, fillers), which show little or no migration capacity, and via the thicknesses of the individual adhesive layers. In the case of adhesive films comprising diffusion barrier layers integrated between the individual adhesive film layers, the concentrations of migratable adhesive film constituents can be adjusted within very wide ranges.
However, the prior art also includes multilayer adhesive films which can be released without residue from the adherends by pulling in the lengthwise direction of the film. However, these films too have grave disadvantages:
Thus WO 92/11333 describes an adhesive tape which is redetachable by pulling in the bond plane and which as backing uses a highly stretchable, substantially nonresilient (nonelastomeric) film having less than 50% resilience after stretching.
WO 92/11332 describes an adhesive tape which is redetachable by pulling in the bond plane and for which it is possible, as backing, to use either a stretchable, highly elastic (elastomeric) film or a highly stretchable, substantially nonresilient film. Adhesive compositions employed are exclusively UV-crosslinked acrylate copolymers.
WO 95/06691 describes adhesive tapes which are redetachable by pulling in the bond plane and which use as their backing a foamed film backing which does not have contact adhesion.
Highly elastic backing materials comprising tackifier resins or plasticizer oils as blend components are not described in WO 11332 or in WO 11333 or in WO 94/21157. Consequently, there is a gap here, which is significant for practical use and limits the application possibilities, for the use of pressure-sensitive adhesive compositions comprising low molecular mass migratable constituents which can migrate into the highly elastic backing materials.
Finally, U.S. Pat. No. 4,024,312 describes highly stretchable adhesive films which possess a backing of highly elastic, thermoplastically processable styrene block copolymers. The backing is coated on at least one side with a pressure-sensitive adhesive composition. The pressure-sensitive adhesives used employ either polyisoprene (e.g. natural rubber) or the styrene block copolymer-based synthetic rubbers which are also employed for the backing material, in a blend with tackifier resins and, if desired, with further blend components. The adhesive tapes can be removed readily from the substrate by stretching parallel to the bond surface. The backing of these adhesive films is described as "basically non-tacky" (col. 1, line 10), although it may include admixed resins. Accordingly, these resins are also described as being those resins which associate with the A blocks of the ABA block copolymers (col. 2, lines 11-16) and are compatible with them (col. 3, lines 10-22). The pressure-sensitive adhesive used can also include resins, i.e. tackifier resins, which associate with the B blocks of the ABA block copolymers (col. 3, lines 58-63).
Although products according to U.S. Pat. No. 4,024,312 may be adequate for some applications, they still have the grave disadvantage for normal practical use that the two types of resin, on account of their relatively low molar masses, migrate, especially at relatively high temperatures, and partition themselves between backing and adhesive composition with the result that it is impossible to obtain constant adhesion properties.
Self-adhesive tapes of this kind do not have constant product properties: mechanical strengths of the backing and properties of the pressure-sensitive adhesive compositions used are irreversibly altered by the diffusion of the resins. Specific formulation and control of the product properties, which is essential for industrial bonds, is therefore impossible.
In the light of this it was an object of the invention to provide adhesive films which are devoid of such disadvantages, or at least do not have them to the same extent, and which nevertheless do not abandon the undisputed advantages of this prior art.